UNS N08367 is often selected for demanding corrosion environments. Yet corrosion resistance is only part of the reason engineers consider this 6Mo superaustenitic stainless steel. Its elevated-temperature strength can also make a meaningful difference in equipment design.
A published comparison at 427°C (800°F) illustrates the point: the allowable stress listed for UNS N08367 is approximately 124 MPa, compared with 66 MPa for Type 316. Why can two austenitic stainless steels perform so differently?
UNS N08367 vs Type 316: the published comparison
| Material | Temperature | Allowable stress in the reference |
|---|---|---|
| UNS N08367 | 427°C / 800°F | 18 ksi / approximately 124 MPa |
| Type 316 stainless steel | 427°C / 800°F | 9.6 ksi / approximately 66 MPa |
Source: IMOA, Practical Guidelines for the Fabrication of Austenitic Stainless Steels, second edition (2020), Section 5.3. The manual presents these figures as an ASME Code comparison. They are a published illustration, rather than a substitute for project-specific design tables.
In this comparison, N08367 has approximately 1.88 times the allowable stress of Type 316—about 88% higher. “Nearly double” is therefore a reasonable description of this particular example, but it should not be generalized to every product form, temperature or code edition. The comparator is Type 316; these figures should not automatically be assigned to 316L.
Allowable stress is not yield strength
Three different properties are frequently confused in material selection:
- Tensile strength: the maximum stress reached during a tensile test.
- Yield strength: the stress associated with the specified onset of permanent deformation.
- Allowable stress: the design stress permitted under the applicable code, taking account of its strength criteria and safety rules.
The 124 MPa figure above is an allowable-stress value. It does not mean that N08367 has a measured yield strength of 124 MPa at 427°C. Before using any design value, check the governing code edition, material specification, product form, condition, thickness and relevant table notes.
Why does UNS N08367 retain a strength advantage?
UNS N08367 is an iron-base, nitrogen-bearing superaustenitic stainless steel, commonly associated with AL-6XN. Despite its UNS “N” designation, it should not be described as a nickel-base alloy.
Compared with conventional 316L, its alloy design combines higher chromium, nickel and molybdenum levels with a deliberate nitrogen addition. These elements support corrosion performance and the desired austenitic structure; nitrogen is also an effective interstitial strengthener. The result is a material whose performance goes beyond simply adding more molybdenum to 316.
All austenitic stainless steels lose strength as temperature increases. A higher starting strength and different alloying response, however, mean that they do not all retain the same strength at elevated temperatures. This helps explain why a corrosion-oriented high-performance grade can also offer a useful design-strength advantage.
Microstructure and thermal exposure still matter
An austenitic matrix does not undergo the same ferrite-related alpha-prime embrittlement mechanism associated with ferritic stainless steels and the ferrite in duplex grades. This is a useful distinction when evaluating elevated-temperature behavior.
It does not make superaustenitic stainless steel immune to thermal damage. High-alloy grades can develop undesirable intermetallic phases, including sigma and chi, under certain thermal exposures. Heat treatment, hot working and welding procedures therefore need careful control. A single allowable-stress comparison cannot establish suitability for prolonged service at a particular temperature.
Higher strength also changes fabrication
A property that benefits a designer can create additional work in the workshop. Compared with a lower-strength austenitic grade, fabrication of N08367 may involve higher forming forces, greater springback after bending, and more demanding straightening or distortion correction.
For forged, rolled or welded components, material selection should therefore be discussed alongside manufacturing capability. Forming equipment, dimensional tolerances, the heat-treatment route and the welding procedure all affect whether the finished component can meet the project requirements.
Does higher allowable stress mean a thinner wall?
It can, when allowable stress is a controlling factor in the applicable design calculation. A higher permitted stress may support a reduced pressure-design thickness or a different pressure rating.
It does not automatically mean that the wall thickness can be halved. Corrosion allowance, minimum manufacturing thickness, external-pressure stability, joint efficiency, mechanical loads and other code requirements can govern the final design. Any potential weight or thickness reduction needs to be demonstrated by the complete calculation.
The purchasing decision should also consider corrosion conditions, fabrication cost, inspection and service life. Type 316 and 316L remain practical choices for many applications. N08367 becomes more attractive when the combination of corrosion resistance and strength justifies its additional material and processing requirements.
What to include in a UNS N08367 enquiry
To help our team review your requirement efficiently, please provide:
- Material grade and governing specification.
- Product form, dimensions, quantity and tolerances.
- Design temperature, pressure and applicable construction code.
- Service medium and relevant corrosion conditions.
- Required heat-treatment condition, inspection and material documentation.
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